Since the invention of the pneumatic tire and its application to motor vehicles such as automobiles, trucks, and the like, many schemes have been proposed to monitor fluid pressure in tires during operation of an associated vehicle. Such systems facilitate maintaining optimum fluid pressure in pneumatic tires which, in turn, reduces the chances of catastrophic loss of vehicle control as well as extending tire life.
A major problem in any tire pressure monitor is the transmission of fluid pressure information from the tire, i.e., a rotating body, to the operator, i.e., a relatively fixed body. Most prior approaches to this problem fall within three general categories. One approach is the direct reading of fluid pressure in a tire wherein sealed, rotating fittings or electrical slip rings are employed in the interface between the wheel and the vehicle body. A second approach is the transmission of fluid pressure information through an inductive or magnetic coupling involving two transducers, one on the wheel and the other on the vehicle body, which are in precise rotational allignment with one another. A third approach is the application of RF transmitters and receivers which use the atmosphere to bridge the interface between the wheel and body. The first two approaches have several shortcomings. First, relatively expensive high-precision components are needed in the slip rings, and inductive-magnetic coupling transducers must remain in alignment at all times during operation of the vehicle. Additionally, both are prone to corrosion and mechanical wear at the point of interface.
Of the tire pressure monitors which have been commercialized, most have followed the third approach and can be categorized as failling into one of two distinct species. The species are distinguished by having wheel mounted units which are either passive or active. The active type employs a transmitter affixed to each wheel which operates in conjunction with a receiver disposed within the vehicle body. These systems tend to be extremely expensive, however, and require a breaking down of the tire from the wheel in order to service and/or replace the battery which powers the transmitter. Additionally, because the information flow was only one way, few of the prior systems have the ability to verify that the system was operating correctly, i.e., had a self-check feature. Finally, due to the dynamic vibration involved in normal vehicle operation, most active prior systems have generally tended to deteriorate over a relatively short period of time, and those that did not suffer rapid deterioration were prohibitively costly for use with private passenger vehicles.
The passive type of transmitter-receiver system includes an interrogating transmitter and receiver on the body of the vehicle and a passive transponding element mounted to the wheel which receives a transmitted RF signal and selectively reflects the signal back to the receiver as a function of the state of a pressure switch in circuit therewith. Most such transponding elements were either of the tank circuit type, comprising a series connected inductor, capacitor and pressure switch in circuit with an antenna or, of the shorted inductor type in which a coil is arranged in series with a pressure switch which effectively grounds the inductor to prevent a reradiation of the received interrogating signal. Although the patent literature is replete with various passive transponder tire pressure monitoring schemes, few if any have been commercially successful. Federal Communications Commission regulations severely limit the power levels of such transmitters. Furthermore, excessive power level is undesirable because of the likelihood of cross talk between nearby vehicles. Too low a power level, however, will provide an insufficient signal-to-noise ratio, and therefore erratic operation.
Finding a compromise solution for these problems has recently become more urgent in light of government and industry interest in the elimination of "spare" tires for cost and weight reasons and substituting "run flat" tires coupled with a tire pressure monitor.
It will be apparent from a reading of the Specification that the present invention may be advantageously utilized with pneumatic tires intended for many different applications. However, the invention is especially useful when applied to relatively high-speed "on-road" motor vehicles and will be described in connection therewith.